1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus for use on an automobile or the like.
2. Description of the Prior Art
There has been known an electrically operated power steering apparatus for assisting the driver of an automobile in turning a steering wheel when the rotation of the steering wheel is transmitted through a pinion to a rack shaft, which is horizontally displaced to cause tie rods coupled to the respective opposite ends of the rack shaft to turn steerable road wheels of the automobile. In the known electrically operated power steering apparatus, the steering torque of a steering shaft coupled to the steering wheel is detected and an electric motor is energized based on the detected steering torque. The rotational power of the electric motor is transmitted through a ball-and-nut mechanism to the rack shaft to assist in turning the road wheels. Such electrically operated power steering apparatus are disclosed in Japanese laid-open patent publication No. 59-50864 and Japanese laid-open utility model publication No. 59-172072.
In the electrically operated power steering apparatus disclosed in Japanese laid-open patent publication No. 59-50864, a ball-and-nut mechanism is positioned substantially centrally on a rack shaft, and the rack shaft is radially supported at five locations including bushings. It is difficult to keep the rack shaft in full coaxial alignment at all of the five supporting locations. Because of alignment errors of the rack shaft, different dimensional and assembling errors of the parts, and flexing displacements of the rack shaft caused by loads applied through tires and tie rods, the ball-and-nut mechanism and a rack-and-pinion mechanism are subject to undue forces tending to displace or deform them or tend to suffer undesirable friction. When this happens, the driver of the automobile has a bad steering feel, and the efficiency of the electric motor of the electrically operated power steering apparatus is lowered.
In the electrically operated power steering apparatus disclosed in Japanese laid-open utility model publication No. 59-172072, a rack shaft is radially supported at two locations, i.e., a ball-and-nut mechanism and a rack-and-pinion mechanism. Since the rack shaft radially supported at two positions is maintained in better coaxial alignment at those supporting positions, it is subjected to less undue forces. However, the rotor of an electric motor of the electrically operated power steering apparatus is supported in a cantilevered fashion due to its structural limitations. As a result, the distal end of the rotor is liable to interfere with an iron core which is fixed to a rack housing.